In general, apparatuses for displaying images are known. An example of an apparatus for displaying images to a user is a head-mounted display system. Head-mounted display systems can be generally referred to as “wearable displays,” because they are supported by a user while in use. Conventional wearable display systems typically include a head-mounted portion, the head-mounted portion having image-generating devices for generating images viewable by the user. Wearable display systems convey visual information, such as data from sensing devices, programmed entertainment such as moving or still images, and computer generated information. The visual information may be accompanied by audio signals for reception by a user's ears. A conventional wearable display system 10 is illustrated by FIGS. 1 and 2.
The conventional wearable display system 10 includes a head-mounted portion 12, which includes right and left image displays 16, 18 and right and left eyepiece optics 20, 22, for displaying images to a user's eyes 21, 23. A controller 25 conveys image signals to the head-mounted portion 12 via a cable 14 having right and left lines 24, 26 for conveying image signals to the right and left image displays 16, 18, respectively. The controller 25 is comprised of an image source 30, which transmits image data to a processor 28, which then formats the data for transmission to the right and left image displays 16, 18.
The right and left image displays 16, 18 are flat panel displays capable of displaying an n×n array of individual points of light, or “pixels.” These types of matrix display devices are commonly referred to as “microdisplays.” The term “microdisplay” describes devices such as liquid crystal displays (LCDs), light emitting displays (LEDs), and scanning devices using cathode ray tubes (CRT's) or laser diodes. FIG. 3 illustrates an n×n array of pixels of a typical microdisplay, with the size of individual pixel elements exaggerated for illustrative purposes. An n×n array such as the one illustrated by FIG. 3 is typically described in terms of its “resolution,” which is measured as the number of pixels per unit area of the display. The n×n pixels illustrated in FIG. 3 are of uniform size. While this configuration is capable of generating a high resolution image, the uniform size of the pixels is inefficient, because the human eye cannot appreciate the relatively high degree of resolution along the edges of the conventional array. This phenomenon is illustrated by FIGS. 4 and 5.
FIG. 4 illustrates a vertical range of vision diagram for a human viewer, the range of vision establishing a human's field of vision. FIG. 5 illustrates the regions of varying visual acuity for a human's left and right eyes while viewing an image together. The inner region 30 of the diagram is a region of relatively high visual acuity. The shaded outer region 32 in the diagram illustrates the portion of a person's field of vision that is of low acuity, but which is also very sensitive to changes in brightness and motion. The outer region 32 includes the peripheral vision. A similar diagram can be constructed for the regions of varying acuity as perceived by a single eye. As illustrated by FIG. 5, the acuity of the eyes decreases with increasing angular deviation (both horizontal and vertical) from the line of sight l. It is therefore undesirable to provide high resolution along the edges of an array, as in the wearable display system 10, because the cost of display devices increases with resolution, and high resolution at the edges of displays provides little viewing benefit to the user of the wearable display system 10. Further, the cost to store, process, and transfer very high resolution signals further increases the cost of the wearable display system 10.
One conventional approach to the above problem is the use of display devices having variable resolution. One such device is disclosed in U.S. Pat. No. 4,479,784 to Mallinson et al. Mallinson's device uses a bifurcated projection system to project an image onto a screen for viewing by a trainee. A region of high resolution imagery is projected onto a center portion of the screen by a foveal projection system, and a region of low resolution is projected by a peripheral projection system. While Mallinson's device accommodates the varying acuity of the user, his system would require large and complex optics and light levels to project into the user's far peripheral viewing region.
U.S. Pat. No. 5,071,209 to Chang et al. discloses a similar device, in which a processor creates images from pixels, with pixel size decreasing with increasing distance from the center of the image. To extend image features into the periphery, Chang's device would require a large and complex optical system to further extend the main microdisplay pixels into the user's far peripheral viewing region.
In U.S. Pat. No. 6,115,007, Yamazaki utilizes a liquid crystal flat panel display having pixels of varying width. Yamazaki, however, utilizes varying pixel width to account for distortion at the horizontal extremes of the viewer's vision. Yamazaki's device also requires complex control architecture to generate images on the flat panel display.
There is therefore a need for a method of displaying images on a wearable display and for wearable display that accommodate the varying acuity of the human eye, and that present a wide field of view to a user.